Conventionally, an image pickup apparatus that picks up images based on light or radiation detected has a light or radiation detector for detecting light or radiation. An X-ray detector will be described by way of example here. An X-ray detector has an X-ray sensitive X-ray conversion layer (X-ray conversion film). The X-ray conversion layer converts incident X rays into carriers (charge signals), and the X rays are detected by reading the converted charge signals. For example, the X-ray detector includes a plurality of X-ray detecting elements arranged vertically and horizontally in a two-dimensional matrix form, charge detecting and amplifying circuits (CSA: Charge Sensitive Amplifiers) for converting charge signals converted by the X-ray detecting elements into voltage signals, signal amplifying circuits for amplifying the voltage signals from the charge detecting and amplifying circuits, and a sample hold circuit for sampling the voltage signals outputted from the signal amplifying circuits, holding the sampled voltage signals and outputting them to an A/D converter.
Further, the X-ray detecting elements include collecting electrodes for collecting the charge signals converted by the X-ray conversion layer based on a bias voltage applied from a common electrode, capacitors for storing the charge signals collected by the collecting electrodes, thin-film transistors (TFT: Thin Film Transistors) acting as switching elements, gate lines for controlling the thin-film transistors from a gate driver, and data lines for reading the charge signals from the thin-film transistors.
When X rays are emitted to a patient M to carry out X-ray imaging, an X-ray image transmitted through the patient M is projected to an amorphous selenium film to generate charge signals proportional to the density of the image in the film. Subsequently, the charge signals generated in the film are collected by the carrier collecting electrodes, and the charge signals collected by the collecting electrodes are stored in the capacitors. Further, the charge signals stored in the capacitors are outputted to the charge detecting and amplifying circuits by switching operation of the thin-film transistors (TFTs).
Although, in such construction, the charge signals accumulated in the capacitors of the X-ray detector are outputted to the charge detecting and amplifying circuits by switching operation of the thin-film transistors, even when the thin-film transistors are in OFF state, the OFF state is imperfect and slight leaks of the charge signals (leakage currents) occur from the capacitors. Where numerous detecting elements are connected to the data lines through which the charge signals are read from the thin-film transistors, the leaks of the charge signals will increase with the number of these detecting elements, thereby increasing the influence imparted to image quality. That is, artifacts will occur due to the leaks of the charge signals, causing a problem of lowering image quality. In order to solve this problem, the thin-film transistors of all the detecting elements connected to each gate line are first switched to ON state successively by control from the gate driver, to collect data (charge signals) from all the detecting elements including the leakage currents. Then, the data (charge signals) from all the detecting elements corresponding to the leakage currents is collected, and the charge signals only for the leakage currents are subtracted from the charge signals from all the detecting elements including the leakage currents, thereby reducing the influence of the leaks of the charge signals from the capacitors (see Patent Documents 1 and 2, for example).
[Patent Document 1]
Unexamined Patent Publication No. 2004-23750
[Patent Document 2]
Unexamined Patent Publication No. 2003-319264